1. Field of the Invention
This invention relates to a device for generating a constant voltage and a method for fabricating the device which are preferably usable for various electronic devices and precision electronic measuring instruments.
2. Description of the Prior Art
Conventionally, a primary voltage standard device has been employed which utilizes a Josephson device. The Josephson device consists of two superconductors, the superconductive electrons of which are interacted one another. As a typical Josephson device can be exemplified a tunnel junction element which is composed of a multilayered structure of superconductor/insulator/superconductor. The Josephson element can exhibit some constant voltage steps at the quantized voltage in the current-voltage characteristic to the ac Josephson effect when a microwave is applied. In this case, the constant voltage steps are determined primary on the frequency of the microwave, so the Josephson device can be utilized in the voltage standard device.
A Josephson element practically usable can be fabricated as follows. First of all, a multilayered structure of niobium thin film (˜100 nm)/aluminum oxide thin film (˜2 nm)/niobium thin film (˜100 nm) is formed, and then, micro-processed. In this case, several ten thousand and more Josephson elements can be integrated. Practically, in the case of fabricating a voltage standard device of several metallic Josephson elements, thousand to ten thousand Josephson elements are connected in series to generate a standard voltage of several volts to several ten volts because one element can generate a voltage of about 1 mV.
As mentioned above, the fabricating technique of the conventional Josephson element using metallic superconductors and the constant voltage-generating technique are established and earn confidence. In the conventional Josephson voltage standard device, however, it is required to arrange several thousand to several ten thousand elements two-dimensionally, so that the arrangement area is increased and it becomes difficult to apply a microwave uniformly on all of the arranged elements. In this point of view, it is desired to decrease the arrangement area and thus, down-size the voltage-generating device itself.